The present invention generally relates to detection of various bandwidths of light and, more particularly, but not by way of limitation, to a photoreceiver assembly and method for processing a photosignal from a high-powered laser system into a plurality bandwidth signals, especially in the high-frequency bandwidth range including, for example an ultrasonic signal.
Typically, a photoreceiver facilitates the conversion of photonic energy into an electrical signal in the bandwidth of interest. In general, current photoreceivers do not provide for sufficient detection in the high-frequency bandwidth range associated with ultrasonic testing of composite materials.
Illustratively, through well-established techniques in metrology, telecommunications photoreceiver systems often extract about 1.0 milliwatt (mW) of power, maybe 10 milliwatts maximum, for high-frequency bandwidth detection and signal processing. As such, instead of sending data signals over optical or electrical channels, metrology provides techniques for extracting an information signal carried along with a typically larger optical signal.
Current photoreceiver systems employ balancing techniques that typically cancel out low frequency signals through phase cancellation so as to extract a single high-frequency signal from the optical carrier. This technique is deficient in that photoreceiver systems must continuously and accurately provide for the complete cancellation of low-frequency signals. Moreover, this technique does not provide the opportunity to concurrently obtain information from low-frequency signals as well as high-frequency signals in that the low-frequency signals are cancelled out. Unfortunately, today""s photoreceiver systems also do not employ metrology techniques to extract ultrasonic signals in the region from 0.5 megahertz (MHz) to 10 megahertz from a larger optical carrier.
As another shortcoming, photoreceiver systems currently do not work for optical signals having very high light levels so as to provide the requisite fidelity to measure small and/or high-frequency signals riding along with a low-frequency signal. As such, increasing the optical energy of an optical signal received by a photoreceiver system increases the signal-to-noise ratio. To enhance the fidelity of the high-frequency band of interest, an increased signal-to-noise ratio is desirable. In general, the signal-noise ratio improves as a net square root of the amount of light collected by a photoreceiver.
Today""s photoreceiver systems, typically within the telecommunications industry, often collect light to provide about 1.0 milliwatt of optical power for use in identifying high-frequency bandwidths of interest. Accordingly, there does not exist a photoreceiver system that provides sufficient power to detect ultrasonic signals in the high-frequency bandwidth of interest. For example, today""s photoreceivers cannot extract an ultrasonic signal in the region from 0.5 megahertz to 10 megahertz off of the optical carrier for that signal.
In accordance with the present invention, a photoreceiver assembly accurately separates at least one low level, high-frequency signal, such as an ultrasonic signal, carried from a large, low frequency pulse. The photoreceiver assembly receives a photosignal from a source, such as a laser system, and produces a photocurrent. This photocurrent is split into two or more separate loops and amplifies each loop within that loop""s transimpedance stage. Therefore, the current-to-voltage conversion process for each resulting voltage signal is optimized for the size of the signal of interest.
Unlike current balancing techniques that are hard to calibrate and require all the signals of interest to be present, the present invention introduces a simplified metrological configuration that uses a photodetector and splitter to isolate various bandwidths of interest. The photoreceiver assembly includes a photodetector and a splitter coupled to the photodetector. The splitter is also coupled to a plurality of transimpedance signal paths. In operation, the splitter receives a current from the photodetector, separates, and directs the current to the plurality of transimpedance signal paths. For example, the splitter extracts a high-frequency ultrasonic signal from a large, low-frequency signal and directs the ultrasonic signal to one path of the plurality of transimpedance signal paths for conversion to a voltage signal via a transimpedance amplifier assembly coupled to the path.
In one exemplary embodiment, the splitter comprises a T-network transformer system. Thus, for a trigger condition, the transformer system sends photocurrent from the primary to the secondary of a transformer so as to separate a high-frequency signal component of the photocurrent from a low frequency component that bypasses the secondary. If the trigger condition is not satisfied, however, the transformer system is a short circuit such that current from the photodetector flows directly to a low-frequency register via a shunt assembly.
It should also be said that in this disclosure and the appended claims, the term xe2x80x9ctrigger conditionxe2x80x9d refers to at least one stimulus based on time and/or physical circumstances. For example, a trigger condition may be a timer sequence or frequency bandwidth condition. Moreover, in one exemplary embodiment, a trigger condition comprises a predetermined circumstance.
A high-frequency unit is coupled to the high-frequency path and includes a forward path gain arrangement and a high-frequency transimpedance amplifier arrangement. The forward path gain arrangement is configured to receive high-frequency current from the transformer system while providing for a large voltage signal beyond the rated voltage requirement for operating the transformer. The high-frequency transimpedance amplifier arrangement converts the high-frequency component of the photocurrent into a signal voltage, such as for example a signal voltage defining the ultrasonic signal. Further along each path a controlled amplifier may be positioned to tune the resulting signal voltage. These controlled amplifiers may be computer-controlled amplifiers in one embodiment, buffer amplifiers in another embodiment, and a combination of a common-mode choke unit and a voltage-controlled amplifier assembly in yet another embodiment.
Ultimately, the photoreceiver assembly provides at least two voltage signals for processing, for example a low-frequency pulse as well as a high-frequency signal extracted from that pulse. For example, in laser systems, the low-frequency pulse may be provided to an interferometer stabilization system comprising a feedback control for maintaining generation of laser induced photosignals in the bandwidth of interest. As another example, an ultrasonic voltage signal may be received for signal processing and image generation of a component part as that part is subjected to laser ultrasound.
Still other intentions, objects, features, and advantages of the present invention will become evident to those skilled in the art in light of the following.